The office building and the bus terminal are the same building. Google maps shows that there could be enough room to fit a track in, albeit the clearance would be tight. That said, would it be really that tight compared to other sites, like what's being proposed at the Davenport grade separation? So I question, again not as a civil engineer or someone experienced with CN requirements for track clearances, if you would actually need to loose that office building.
Looking again:
You could also just raze that south platform, and put a track in its place instead -- and shift the northern tracks instead to permit a center platform. This would require bridge rebuilds but would keep more distance between the southern track and the building. With a small noisewall and anti-vibration built into the railbed of a new rail bridge, you might even have similar or less vibration as today, in the existing office building, and you'd have those stairs (the south-east platform access stairs, between building and tracks) relocated into a pedestrian tunnel that leads up into a center platform.
It does mean putting a slight curve into some of the tracks, but this seems to be a "which track do I want to put a slight curve into?" pick-your-poison scenario.
I suppose we will be stuck with 3 tracks, but it would be interesting to run this scenario of simply razing the existing south platform to put a track there in its place, and use a center platform (north of southernmost track) and a north platform (with perhaps 1 through express track, 2nd from northmost). This would mean 3 berthable tracks and 1 through track, with manageable curves.
Another way to view this is you're trading space between the south platform and the southernmost track -- combined with a slight shift -- to allow one platform to access two tracks, and keep the north platform (which will need to move further northwards) from needing to become a centre platform & prevent worsening track curves to the north in a 4-track scenario.
Pros:
- 4 track corridor
- similar clearance to existing office building (track in place of south platform)
- eliminates need to build a 4th track literally flush to office building
- express track (good for freight trains, HSR or express trains)
Cons:
- complex station rebuild & service disruptions due to platform shift AND track shift
- you'll have platforms at only 3 tracks, due to trying to limit curve for northernmost track
- need of anti-vibration to compensate for tracks slightly closer to office building
I may have missed something (additional unexpected expropriations beyond those mentioned, etc) caused by curve encroachments, but if they are possibly rebuilding the station anyway, is this a feasible "compromise" scenario at all for a theoretical 4-track corridor?